The field of the present invention is speed reduction transmissions applicable to machinery devices in, for example, robots, automobiles, machine tools and electrical tools.
In a wide variety of applications, it is desirable to reduce the speed of a rotary output using a power transmission system. Two such systems are commonly known, a harmonic drive system and a Ferguson drive system. Both offer a high speed reduction ratio. The harmonic drive system, as shown in FIG. 1, is such a device which provides substantial speed reduction employing an elliptically shaped wave generator rotatably driven within a ring gear. A ring gear 1 with inner teeth surrounds an elastic gear 2 rotatably arranged outwardly of an elliptical shaft 3. The elliptical shaft 3 is driven to generate a wave in the elastic gear with the elastic gear engaged diametrically with the teeth of the ring gear 1. The elastic gear 2 is caused to rotate by this system in the opposite direction to the wave generator 3 at a speed dependent upon the difference in the number of teeth between the elastic gear 2 and the ring gear 1. Such a harmonic drive system is difficult to construct, requiring a high level of manufacturing technology.
The Ferguson drive system as illustrated in FIGS. 2 and 3 also carries out significant speed reduction. The sun gear 5 is driven. The planetary gears 6 rotate and revolve about the sun gear 5. The ring gear 7 is fixed. The ring gear 8 provides the output. The ring gears 7 and 8 have different numbers of teeth and the output speed of the ring gear 8 depends upon this difference. The Ferguson drive system has substantial gear wheel diameter disparity which can be disadvantageous, particularly as to the noise generated.